1. Field of the Invention
The present invention relates to a fuel cell system and a scavenging method thereof.
Priority is claimed on Japanese Patent Application No. 2007-023480, filed Feb. 1, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a known fuel cell, an anode and a cathode are provided on either side of a solid polymer electrolyte membrane so as to form a membrane electrode assembly. The membrane electrode assembly is placed between a pair of separators, thereby forming a unit fuel cell (called a “unit cell” below) having a plate shape. Generally, a specific number of unit cells are stacked so as to form a fuel cell stack (called a “fuel cell” below).
In this fuel cell, a hydrogen gas is supplied between the anode and the corresponding separator, while air as an oxidant gas is supplied between the cathode and the corresponding separator. Accordingly, hydrogen ions generated at the anode by catalytic reaction move through the solid polymer electrolyte membrane to the cathode, and an electrochemical reaction occurs between the hydrogen ions and oxygen at the cathode, thereby generating electric power. In this process, water is generated in the fuel cell.
When the temperature of the fuel cell falls below the freezing point while the electric-power generation stops, water remaining in the fuel cell freezes. Accordingly, the function of the membrane electrode assembly is degraded, which also degrades starting (or activation) performance or power-generating performance of the fuel cell.
Japanese Unexamined Patent Application, First Publication No. 2002-208422 discloses a method of supplying a dry gas to at least one of an air passage and a hydrogen passage after the normal operation of the fuel cell is stopped; making the dry gas capture water in the fuel cell; and discharging the gas as a wet gas.
In a technique which has been proposed in recent years, air, which is supplied as a reaction gas to a cathode gas passage, is also supplied to an anode gas passage as a scavenging gas. In this case, a scavenging switching valve is provided for switching the target, to which air is supplied, between the cathode gas passage and the anode gas passage.
However, when the scavenging switching valve is switched so as to perform anode scavenging after completion of cathode scavenging, a pressure difference occurs between both sides of the scavenging switching valve. That is, the upstream side of the scavenging switching valve has a higher pressure while the downstream side thereof has a lower pressure. In this case, a large amount of power (or force) is required for opening the scavenging switching valve. If a scavenging switching valve having a large amount of opening and closing power is employed so that the valve can be opened against a large pressure difference, then the size of the scavenging switching valve is increased, thereby increasing the cost of the system.